Back pain, but haven't gained a lot of weight

Mommy Of 2
My lower back has been killing me lately, but I haven't put on more than 8 lbs max (and I'll be 31 weeks in the next day or so). I'm even wearing all of my same prepregnacy tshirts, so I've not gained a huge amount, although I am definitely showing. So I don't know how much of it is due to weight gain.
Has anyone else had lower back pain? What did you do about it? It is killing me. 😭😭